A semiconductor wafer typified by a silicon wafer is polished with a polishing apparatus 601, as shown in FIG. 6, including a turn table 603 with a polishing pad 602 attached, a polishing-agent-supply mechanism 604, and a polishing head 606 by holding the semiconductor wafer W with the polishing head 606, supplying a polishing agent 605 from the polishing-agent-supply mechanism 604 to the polishing pad 602, separately rotating the turn table 603 and the polishing head 606, and bringing the surface of the semiconductor wafer W into sliding contact with the polishing pad 602.
The semiconductor wafer is often polished in multiple stages with changing the kinds of the polishing pad and the polishing agent; first double-side polishing is referred to as primary polishing, polishing after the primary polishing is referred to as secondary polishing, and a polishing process in the final stage is referred to as finish polishing or final polishing.
In the final polishing process, single-side polishing is often employed, and the polishing agent and the polishing pad are selected so as to reduce surface defects and surface roughness of a wafer, called haze. The materials typically used are an alkali-based polishing agent containing colloidal silica and a suede type of polishing pad. Since a polished wafer has a hydrophobic surface, foreign substances are attached to the surface, and the surface is non-uniformly etched by alkali in the polishing agent, which causes haze unevenness. It has thus been known that a water-soluble cellulose such as hydroxyethyl cellulose is added to the polishing agent to make the surface of the polished wafer hydrophilic and inhibit attachment of foreign substances and etching unevenness on the surface.
With respect to the haze of a wafer, addition of hydroxyethyl cellulose (HEC) to the polishing agent has been known to more significantly reduce a haze level than addition of other cellulose (See Patent Document 1, for example). However, hydroxyethyl cellulose, which is suited to improve hydrophilicity of a wafer, is difficult to be filtered and easy to aggregate, thus disadvantageously increasing micro defects on the wafer surface. To solve this problem, it has been proposed that hydroxyethyl cellulose having a low molecular weight is used to improve the filterability, or an undiluted solution is filtered, and then a diluted solution is filtered again (See Patent Document 2, for example). Moreover, it has been proposed to control the ratio of average secondary particle sizes of abrasive grains before and after dilution (See Patent Document 3, for example).